This invention relates to in-situ remediation of contaminated soils. In one aspect, this invention relates to a novel process combining electrokinetics and treatment of contaminants using phytoremediation. In a further aspect, this invention relates to a novel process for the in-situ remediation of soils contaminated with toxic organic compounds and/or ionic contaminants.
Generally, degradation of toxic organic compounds to innocuous products such as CO.sub.2 and water can be accomplished either biologically or physicochemically provided the treatment is carried out in a well-controlled environment in which key operating parameters such as temperature, pressure, mixing, addition of the reactants or nutrients, etc. are optimized. Examples of these technologies include incineration and its variations, supercritical water oxidation, UV/H.sub.2 O.sub.2 /ozonelcatalytic oxidation, reductive dehalogenation and biodegradation in an optimized bioreactor. However, the costs associated with these technologies are high for the decontamination of soil, which must first be excavated and then processed into a form suitable for the particular reactor used. The reactor constitutes a major portion of the overall cost in these processes due to either the extreme conditions required with thermal approaches or the very long holding times required in biological approaches. To overcome these problems, destruction of the contaminants needs to be done in-situ to avoid the cost and complications associated with excavation and handling, and the process has to be energy efficient and mild to minimize capital and operating costs.
Various techniques have been suggested for application in processes for the in-situ remediation of soils contaminated with toxic organic compounds. Examples of such techniques include hydraulic fracturing, also referred to as hydrofracturing, and electroosmosis. However, these techniques as currently practiced suffer from limitations which make them commercially impractical.
Hydraulic fracturing is an established oil field technology for increasing the production rates of oil or gas wells which has recently been adapted by the Environmental Protection Agency (EPA) Risk Reduction Engineering Laboratory as a method to access subsurface soils for remediation purposes. See EPA Groundwater Currents, Office of Solid Waste and Emergency Response Technology Innovation Office, September 1992. While this technique is of little utility as a remediation technique by itself, it has potential for enhancing other remedial technologies such as vapor extraction, steam stripping, soil washing, and especially bioremediation. A major problem with the use of hydraulic fracturing, however, involves its use with contaminated fine-grained soils such as clayey or silty soils. These soils have such low permeabilities that it is not possible for liquids to be pumped through uniformly by hydraulic means. Therefore, contaminants in these soils remain poorly accessible.
Electrokinetics, specifically electroosmosis, is another technique which has been suggested for use in in-situ remediation of soils contaminated with non-ionic, soluble organic compounds. Electroosmosis involves applying an electrical potential between two electrodes immersed in soil to cause water in the soil matrix to move from the anode to the cathode when soils are negatively charged, such as is the case with clayey soils. When the soil is positively charged, however, the direction of flow would be from the cathode to the anode. The technique has been used since the 1930's for removing water from clays, silts and fine sands. The major advantage for electroosmosis as an in-situ remediation method for difficult media, e.g. clay and silty sand, is its inherent ability to get water to flow uniformly through clay and silty sand at 100 to 1000 times faster than attainable by hydraulic means, and with very low energy usage. Electroosmosis has two major limitations as currently practiced that makes it impractical for actual field remediation. First, the liquid flow induced by electroosmosis is extremely slow, i.e. about one inch per day for clayey soils, which could result in a cumbersome and very long-term operation in large-scale operations. Second, several laboratory studies (see Bruell, C. J. et al., "Electroosmotic Removal of Gasoline Hydrocarbons and TCE from Clay", J. Environ. Eng., Vol. 118, No. 1, pp. 68-83, January/February 1992 and Segall, B.A. et al., "Electroosmotic Contaminant-Removal Processes", J. Environ. Eng., Vol. 118, No. 1, pp. 84-100, January/February 1992) have indicated that part of the soil bed became dry after approximately one month under the electroosmotic effect, resulting in reduced flow and the eventual stoppage of the process. Another laboratory study (see Shapiro, A. P. et al., "Removal of Contaminants From Saturated Clay by Electroosmosis", Environ. Sci. Technol., Vol. 27, No. 2, pp. 283-91, 1993) has indicated that the acid generated at the anode moves through the soil bed in the direction of the cathode and results in reduced electroosmotic flow and eventual stoppage of the process.
Electrokinetics, specifically electromigration, involves applying an electrical potential between two electrodes immersed in soil to cause solute, e.g. ions of metals, to migrate through a solution along the imposed voltage gradient, i.e. electromigratory movement. The charged species of metals in the soil migrate toward the oppositely charged electrodes and are collected at the electrodes. Electromigration has several limitations as currently practiced that make it difficult for actual field remediation. First, pH of the solution near the cathode tends to be very alkaline due to water electrolysis at the electrode and this causes most metals to precipitate in the soil making it difficult to remove the contaminants as well as blocking the flow of water through the contaminated soil region. Second, electrokinetics is inherently not a very stable process due to build-up of concentration, pH and osmotic gradients in the soil between the electrodes which adversely affect the process. In addition, the soil itself will also be altered over time, e.g. the soil will suffer from drying and cracking.
A process for the in-situ remediation of contaminated soil which combines electrokinetics with treating zones containing material for treating contaminants is disclosed in U.S. Pat. No. 5,398,756, which patent is incorporated by reference herein. U.S. Pat. No. 5,398,756 solves the above-described problems with electrokinetics.
Phytoremediation, i.e. remediation using plants, has been shown to be locally effective for uptake of metals from soil and water. Less is known for organic contaminants, especially those recalcitrant organic compounds that have very low water solubility and strong binding to the soil (high log K.sub.ow), i.e. polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs) and dioxins. A major contributing factor to the persistence of recalcitrant contaminants is that only a few species of microorganisms appear capable of degrading those compounds. The challenge in bioremediation of such compounds is to promote the growth and activity of the small minority component of the indigenous microbial community that effectively biodegrade these compounds. One approach is to provide co-metabolites from plant roots to promote the growth and activity of the effective indigenous microorganisms. However, phytoremediation suffers from problems like the seasonal nature of the production of the co-metabolites, and the localized effect of the co-metabolites, i.e. only those contaminants in the immediate vicinity of the roots are effected.
An in-situ remediation process for single or mixed waste contamination remediation which is commercially practical and economical, and solves the above-described problems with phytoremediation would be highly desirable. A process has now been discovered which combines electrokinetics and phytoremediation to degrade contaminants, particularly recalcitrant organic compounds, solves the above-described problems with phytoremediation.